class 8 ch 13

Chapter 13. SOUND |Science | CBSE Class VIII (8th) | Solved Exercises

Things to remember....

1. Sound is produced by vibrating objects.
2. In human beings, the vibration of the vocal cords produces sound.
3. Sound travels through a medium (gas, liquid or solid). It cannot travel in vacuum.
4. It sends the signals to the brain. This process is called hearing.
5. The number of oscillations or vibrations per second is called the frequency of oscillation.
6. The frequency is expressed in hertz (Hz)
7. Larger the amplitude of vibration, louder is the sound.
8. Higher the frequency of vibration, the higher is the pitch, and shriller is the sound.
9. Unpleasant sounds are called noise.
10. Excessive or unwanted sounds lead to noise pollution. Noise pollution may pose health problems for human beings.
11. Attempts should be made to minimise noise pollution.
12. Plantation on the roadside and elsewhere can reduce noise pollution.

Question 1. Choose the correct answer :
Sound can travel through:

1. gases only
2. solids only
3. liquids only
4. solids, liquids and gases.

Answer : d. solids, liquids and gases.

Question 2. Which of the following voices is likely to have minimum frequency?

1. Baby girl
2. Baby boy
3. A man
4. A woman

Answer : c. A man

Question 3. In the following statements, tick T against those which are true, and F against those which are false: 

1. (a) Sound cannot travel in vacuum. (T/F)
2. (b) The number of oscillations per second of a vibrating object is called its time period. (T/F)
3. (c) If the amplitude of vibration is large, sound is feeble. (T/F)
4. (d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T/F)
5. (e) The lower the frequency of vibration, the higher is the pitch. (T/F)
6. (f) Unwanted or unpleasant sound is termed as music. (T/F)
7. (g) Noise pollution may cause partial hearing impairment. (T/F)

Answer : 

1. (a) Sound cannot travel in vacuum. (T)
2. (b) The number of oscillations per second of a vibrating object is called its time period. (F)
3. (c) If the amplitude of vibration is large, sound is feeble. (F)
4. (d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T)
5. (e) The lower the frequency of vibration, the higher is the pitch. (F)
6. (f) Unwanted or unpleasant sound is termed as music. (F)
7. (g) Noise pollution may cause partial hearing impairment. (T)

Question 4. Fill in the blanks with suitable words. 

1. (a) Time taken by an object to complete one oscillation is called ................... .
2. (b) Loudness is determined by the .........................of vibration.
3. (c) The unit of frequency is..........................
4. (d) Unwanted sound is called ......................... .
5. ((e) Shrillness of a sound is determined by the ................. of vibration.

Answer : 

1. (a) Time taken by an object to complete one oscillation is calledTime Period
2. (b) Loudness is determined by the Amplitude of vibration.
3. (c) The unit of frequency is Hertz (Hz)
4. (d) Unwanted sound is called Noise Pollution
5. ((e) Shrillness of a sound is determined by the Frequency of vibration.

Question 5. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.

Answer : 

We know that the number of oscillations per second is called the frequency of oscillation.
∴The frequency (F) of oscillating pendulum	=The number of oscillations/ Time taken in Seconds
	= 40/4 Hz = 10 hz
	= 10 hz
Also, We know, the Time period (T)is the total time taken for making one complete oscillation
∴ Time Period of oscillating pendulum	= 1/10 seconds
	= .1 second

Question 6. The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration?

Answer : 

The frequency (F) of Mosquito vibrations =	500 Hz
Also, We know, the Time period (T) is the total time taken for making one complete oscillation
∴ Time Period of Mosquito vibration	= [1 /frequency (F) ] seconds
	= (1/500) Seconds
	=.002 Seconds

Question 7. Identify the part which vibrates to produce sound in the following instruments: (a) Dholak (b) Sitar (c) Flute

Answer : 

	instruments	Vibrating Part Producing Sound
(a)	Dholak	Stretched membrane
(b)	Sitar	Stretched string
(c)	Flute	Air-column Variation

Question 8. What is the difference between noise and music? Can music become noise sometimes?

Answer : All excessive and Unpleasant sounds, which may cause discomfort are called noise. The sound of moving vehicles, construction work, Crackers, blaring horns of buses and trucks, loud music are examples of noise, where as the sounds which are pleasant to the ear and one can enjoy them on hearing is termed as Music . Sound produced by a harmonium is a musical sound. The string of a sitar also gives out a musical sound. If a musical sound becomes too loud, it will not be pleasant to hear. it would not remain melodious, rather It will become a noise or a noise pollution

Question 9. List sources of noise pollution in your surroundings.

Answer : 
List sources of noise pollution in your surroundings. 

1. (1.) Sounds of vehicles
2. (2.) Explosions including bursting of crackers,
3. (3.) Running of machines
4. (4.) Loudspeakers, Television and transistor radio at high volume
5. (5.) kitchen appliances such as grinder
6. (6.) desert coolers
7. (7.) Air conditioners

Question 10. Explain in what way noise pollution is harmful to humans.

Answer : The noise pollution is harmful to humans in many ways as it may cause : 

1. (1.) Lack of sleep
2. (2.) hypertension (high blood pressure)
3. (3.) Anxiety
4. (4.) Mood Disorders leading to depression
5. (5.) Temporary or even permanent impairment of hearing.

Question 11. Your parents are going to buy a house. They have been offered one on the roadside and another three lanes away from the roadside. Which house would you suggest your parents should buy? Explain your answer.

Answer : I will suggest my parents to buy the house three lanes away from the roadside because :

1. (1.) There will be less noise pollution caused by the vehicular traffic and crowd
2. (2.) There will be less air pollution caused by the vehicular traffic
3. (3.) There will be additional safety from roadside accidents on account of less traffic
4. (3.) Community based activities or functions are easy to conduct in lanes away from roadside, thus enhancing overall quality living

Question 12. Sketch larynx and explain its function in your own words.

Answer :

In humans, the sound is produced by the voice box or the larynx. It is a hard bump like structure deep inside the throat, located at the upper end of the windpipe. Two vocal cords, are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for the passage of air.
When the lungs force air through the slit, the vocal cords vibrate, producing sound. Muscles attached to the vocal cords can make the cords tight or loose. When the vocal cords are tight and thin, the type or quality of voice is different
Question 13. Lightning and thunder take place in the sky at the same time and at the same distance from us. Lightning is seen earlier and thunder is heard later. Can you explain?

Answer : The light travels at a speed of 300000000 m/s where as sound travels at a speed of 340 m/s. 

Due to this reason, at the event of a lightening and thunder, taking place in the sky at the same time and at the same distance from us, lightning is seen earlier and thunder is heard later